Object movement is commonly considered to occur in six degrees of freedom, these being linear movement along three perpendicular axes X, Y, and Z, and rotation about these three axes. A number of machines, such as milling machines and vibration testing machines, utilize a workpiece-holding table which must be moved in a limited number of degrees of freedom and which must be prevented from movement in other degrees of freedom.
One approach to table mounting is the cascade approach, sometimes called the "table-on-table" system, which is used in milling machines. In this approach, the workpiece-holding table is supported on a second table having a pair of parallel guides that prevent movement in five degrees of motion and allow movement in only one X-axis direction. The second table is supported on a third table that prevents movement in all but one Y-axis direction, etc. Additional intermediate tables can be used that each prevent movement except rotation about one axis. The main disadvantages of the cascade system are its large weight and the high costs of the numerous bearings.
Another approach to table mounting is the coupled approach, which is the most widely used approach in vibration tables used for seismic testing. Such a system may use a pair of synchronized hydraulic actuators to produce vertical motion without tilting of the table about a particular horizontal axis. Undesired rotational and linear translations are prevented by the use of horizontal links with outer ends pivotally connected to the table and inner ends pivotally connected to a reaction mass. While a coupled system is relatively simple, it has several disadvantages, including the need for a large reaction mass due to the horizontally extending links, the presence of a low natural frequency of vibration for a tall specimen due to compressibility of hydraulic fluid in the two actuators, and the sometimes undesirable arcing motion of the table due to the pivotally mounted links.
An object moving system which was especially simple, required a minimal reaction mass, had a low natural frequency, and avoided considerable arcing movement, would be of considerable value in a variety of applications, especially in the construction of vibration tables.